1. Field of the Invention
The present invention relates to optical discs for data storage. More specifically, the present invention provides an optical disc having either an optical variable device such as a hologram and/or a varying color, depending upon the angle at which it is viewed, to identify the disc as an original and not a counterfeit.
2. Description of the Related Art
Optical discs are increasingly used for data storage, including computer software. Presently available optical discs include pre-recorded and recordable versions of compact discs (CDs) and digital versatile discs (DVDs). Future pre-recorded and recordable optical disc formats have also been announced including Super Audio CD (SACD), HD-DVD and Blu-ray (BD) which have similar disc construction and similarly lend themselves to the inventions and applications mentioned herein.
CDs of 80 mm and 120 mm diameter and 1.1-1.3 mm thickness construction are typically made by first producing a transparent plastic disc (typically polycarbonate), having a single layer or pattern of indentations on one side. The indentations (“pits”) will have similar depth and width, but will have varying discrete circumferential lengths. The pits will be arranged into a single spiral track leading from an inner diameter of the disc to the outer diameter such that the track spacing between successive turns of the spiral track is approximately 1.6 microns. The side of the CD having the indentations is coated with a thin film metal layer having good light reflective properties, for example, aluminum, by well-known methods including vacuum coating or sputtering. Next, a protective covering such as a transparent lacquer is applied over the reflective layer, with the protective layer being cured by exposure to ultraviolet light and a printed label is applied over the protective lacquer. The information on the CD is read by focusing a laser onto the indentations through the underside of the disc (opposite side of the printed label), so that the laser passes through the polycarbonate, and is reflected back through the polycarbonate by the reflective layer, such that the reflected beam can be analyzed to determine the characteristics of the indentations and the data stored therein.
DVDs have nominally the same physical dimensions as CDs but are constructed from two approximately 0.6 mm thick plastic disc component substrates bonded together with the data surfaces adjacent to and opposing each other at the bonding surfaces. Each 0.6 mm component substrate is able to contain up to two discrete layers of indentations applied to the top surface; in the case that two layers of indentations are present on one component disc substrate, the spacing is approximately 40-70 microns in the plane of the disc surface. As a result, DVDs may be constructed with up to two discrete data surfaces on each component disc substrate and data is readable from one or two sides of the disc; this gives rise to a family of named DVD disc constructions related to the “layers” of data storage capacity and the read format. DVD5 is single-sided read single-layer data surface construction; DVD9 is single-side, dual-layer data construction; DVD10 is single-layer, dual-side; DVD14 is dual-side, one single-layer and one dual-layer component; DVD18 is dual-sided dual-layer construction. DVD introduces the concept of dual-layer data readout in the DVD9, DVD14 and DVD18 products; this is achieved by applying a semi-reflective thin film coating to the outer data surface followed by an opaque (fully reflective) thin film metal coating to the adjacent innermost data surface on the dual-layer disc.
BD disc products are of similar physical construction to CD discs although the data surface is read through the protective lacquer “cover” layer of approximately 100 microns thickness. HD DVD disc products are of similar physical construction to DVD discs.
The frequency of unlawful copying of optical discs (and other storage media), combined with the passing of these copies as lawful copies for which the proper royalties were paid to the copyright holder, results in the need for a means for distinguishing an authentic lawful disc from an unlawful copy or counterfeit.
N. C. Abraham's U.S. Pat. No. 5,452,282 discloses a process for producing an optical data storage disc having both a digital recording and a holographic image. The digital recording occupies one portion of the disc, and the hologram occupies any portion of the disc that is separate from the data. The hologram may therefore be located on the outer circumference of the disc, or on the inner portion of the disc, or at any other location where data is not present. The hologram will typically be on the same layer as the data.
N. C. Abraham's U.S. Pat. No. 5,533,002 discloses an optical disc having one side from which the digital data may be read, and a second side that is embossed with a relief pattern defining a holographic image. After applying a layer of metal over the side of the transparent polycarbonate disc containing the indentations, along with the UV-cured protective lacquer, the lacquer coating is embossed with a hologram using heat and pressure. Alternatively, the hologram may be produced within the lacquer by casting. The embossed lacquer layer is then covered with another metal layer, followed by a final UV-cured lacquer layer. The data may thereby be read from one side of the disc, through the transparent polycarbonate and the hologram will be visible on the other side of the disc, through the transparent lacquer.
N. C. Abraham's U.S. Pat. No. 6,160,789 discloses an optical storage disc. The disc is formed using a first polycarbonate disc having a relief pattern defining digital data, having a reflective coating over the relief pattern, and a second polycarbonate disc having a relief pattern defining a hologram, also covered by a reflective coating. The two reflective coatings are adjoined by an adhesive layer therebetween, resulting in a disc that is readable from one side and having a hologram that is viewable from the other side. In other embodiments, the relief pattern is formed within a lacquer coating applied to a flat surface of the second disc. Still other embodiments utilize a plurality of layers having digital data, in the same manner as a DVD.
International patent application number PCT/US00/10985 (publication no. WO 01/03945), published on Jan. 18, 2001, discloses an optical coating having multiple layers of dielectric, and possibly a layer of metal, which produces an observable color shift as the angle of incident light or the viewing angle changes. This patent is primarily directed towards the use of the color-shifting layer in conjunction with a hologram and does not teach that such a color-shifting coating may be used as a reflective, data-containing layer, in an optical disc.
It is desirable to provide a data storage optical disc providing a color that changes according to the angle at which the disc is viewed, and in some embodiments, providing an optical variable device viewable from one side. It is further desirable to produce a disc having the appearance of a changing color with respect to the angle at which it is viewed, when viewed from the readable side of the disc. Such a disc would be particularly difficult to duplicate by those who would counterfeit the information stored thereon. Furthermore, a surface having a different color depending upon the angle at which it were viewed would be usable as a security measure for both single-side and double-sided discs.